1. Field of the Invention
The field of the invention is reduction of NOx, and particulate matter (PM) emissions from Diesel-cycle engines. The field of application is primarily in internal combustion engines for motor vehicles.
2. Prior Art
The growing use of Diesel-cycle engines in motor vehicles greatly adds to the atmospheric presence of pollutants such as oxides of nitrogen and particulate matter. Conventional Diesel-cycle engines emit nitrogen oxide (NOx) and particulate matter (PM) substantially in excess of the emissions from Otto-cycle (e.g., gasoline) engines, yet Diesel-cycle engines achieve substantially better fuel economy. Because of the higher fuel economy, Diesel-cycle engines dominate the heavy-duty truck market and much of the off-road commercial vehicle market, with growing penetration into light duty trucks. Thus, technology which could substantially reduce NOx and PM emissions from Diesel-cycle engines is highly desired.
Two key features of Diesel-cycle engines are the absence of substantial throttling of the intake charge (i.e., air or a mixture of air and recirculated exhaust gas) and the direct injection of fuel into the combustion chamber. A third important feature of most modem Diesel-cycle engines is a turbo charger, usually followed by a charge cooler, to supply pressurized intake charge to allow increased specific power output. The turbo charger usually includes a turbine compressor driven by an exhaust gas turbine expander. During a command for a rapid rise in engine torque, increased fuel can be supplied almost instantaneously. However, if the engine is currently operating with high exhaust gas recirculation (EGR), there is a reduced quantity of oxygen available which will not allow maximum fuel injection without poor combustion and increased PM emissions. Also, until the exhaust energy level is increased to the level associated with the higher torque output, the turbo charger is unable to supply the increased boost pressure (and hence more mass of oxygen) that will ultimately be available at the new equilibrium (commonly called xe2x80x9cturbo-lagxe2x80x9d), and again a constraint must be placed on the maximum fuel injection quantity until the system responds with an increased mass of oxygen.
With conventional technology, it is especially difficult to quickly adjust the quantity of exhaust gas entering the combustion chamber with the charge air, because: (1) the response time of the EGR flow control valve is relatively long compared to the combustion cycles of the engine, and (2) the time required to xe2x80x9cpurgexe2x80x9d the previously desired exhaust gas and air mixture from the intake system through the engine is also relatively long and may take several combustion cycles before the newly desired mixture can be established.
Accordingly, an objective of the present invention is provision of an advanced EGR system to minimize NOx and PM emissions from a Diesel-cycle engine while maintaining or enhancing transient and steady-state performance and durability of such engines.
Another objective of the present invention is to shorten the response time for EGR.
The present invention provides a diesel-cycle engine having a novel exhaust gas recirculation system. More specifically, the present invention is directed to a diesel engine having a plurality of cylinders which define respective combustion chambers therein with fuel feed, e.g. fuel injectors, for feeding successive fuel charges to each of the combustion chambers. An air-intake line receives intake air and feeds it to an intake manifold which distributes the received intake air to the various cylinders for combustion of the fuel charges therein with generation of exhaust gas. A gas turbine is provided in an exhaust line which receives exhaust gas from an exhaust gas manifold which, in turn, collects exhaust gas from the various combustion chambers. An intake compressor, driven by the gas turbine compresses the intake air. A sensor is provided in the exhaust line for sensing concentration of at least one exhaust component in the exhaust gas and an engine controller generates a control signal in accordance with the sensed concentration. A portion of the exhaust gas is recirculated through an exhaust gas recirculation line for feed to the combustion chambers and an exhaust gas cooler is located in the exhaust gas recirculating line for cooling the recirculated portion of the exhaust gas and for separating condensate and particulate matter (PM) therefrom. Optionally, a separate PM filter may be provided in the exhaust gas recirculation line. A return line connects the exhaust gas cooler to the exhaust gas line for discharge of the condensate and particulate matter through the exhaust gas line which vents to the ambient atmosphere. A control valve serves to control flow rate of the recirculated portion of the exhaust gas responsive to the control signal received from the engine controller.
In several preferred embodiments the exhaust gas recirculation line connects the exhaust line, at a point downstream of the turbine, with the air-intake line upstream of the intake-compressor. In another embodiment the exhaust gas recirculation line does not connect with the air-intake line but, rather, delivers exhaust gas, compressed by an auxiliary compressor, to an auxiliary manifold for distribution into the plural cylinders, separate from the intake air introduced through the intake manifold.
Thus, the present invention achieves its objectives by a unique design and means of operation which maintains closed loop control of the fuel injection quantity and/or the EGR quantity. The closed loop control is achieved by measuring a component (or components) of the exhaust (or intake) that correlates well to the level of NOx and/or PM, and by adjusting the fuel quantity injected and/or the quantity of EGR accordingly, to minimize the formation of NOx and PM emissions. The measured components may include but are not limited to oxygen (O2), NOx and/or PM directly, and/or carbon dioxide (CO2). The goal is to use as much EGR as possible for the torque being commanded, and to control the fuel injection quantity to minimize PM formation, especially during engine transients.
The present invention quickly adjusts the quantity of exhaust gas entering the combustion chambers. In one embodiment, a quick EGR response is achieved by providing a separate exhaust gas intake manifold with ports near the combustion chamber intake valves, and thus the delay is only associated with the response of the EGR valve. In another embodiment a quick EGR response is achieved by providing a separate air-only intake manifold with ports near the combustion chamber intake valves and a fast response compressor which provides a pressurized air flow to displace some or all of the in-place air/EGR mixture, thus providing reduced emissions while improving the engine torque rise rate and engine performance.
EGR can be achieved by taking the exhaust gas from the exhaust pipe before the turbo charger turbine expander (called a high pressure system, and this is the most common approach) or after the expander (the low pressure system). There are advantages and disadvantages of both approaches. The low pressure system has advantages of: (1) receiving a lower temperature exhaust gas, (2) simplified control valve, (3) less detrimental impact on the turbo charger performance, and (4) good mixing of the EGR and air. The concerns with current low pressure systems are: (1) slower EGR response time (more intake gas volume to purge), (2) the EGR must be pumped back to a high enough pressure so that it will flow into the pressurized intake, (3) exhaust fouling of the pump (whether the pump is the turbo charger compressor or a separate pump), (4) lower efficiency of the pump because of the higher temperature of exhaust gas as compared to that of ambient air, and (5) the fouling of the charge air cooler. The high pressure system has advantages of: (1) faster EGR response time, (2) no pump/compressor or charge air cooler fouling, (3) a separate EGR cooler can be maintained at a higher temperature than the charge air cooler to minimize the fouling due to condensate, and (4) a generally simpler hardware approach. The concerns with high pressure systems include: (1) more difficult air and EGR mixing, (2) higher final charge gas temperature and therefore lower efficiency and higher NOx, (3) detrimental impact on the turbo charger system, and (4) the EGR. control valve is at a higher temperature and is more complex.
The EGR design of the preferred embodiments of the invention is of the low pressure system type, but several unique new features mitigate the previously identified concerns. First, the system has a very fast response. The EGR pump is driven by the turbo charger turbine expander, and may be either a second compressor wheel or the existing air intake compressor, since the power available from the turbine expander (turbine) is not diminished by removing high pressure exhaust gas.
In the present invention the fouling of the pump and charge air cooler (or separate EGR cooler) and the reduced efficiency of the pump are mitigated by a new design which takes the EGR from a point downstream of the turbine to make use of normal exhaust gas cooling, returns the exhaust gas to a location near the pump with the return tubing serving to further cool the EGR, and routes the partially cooled EGR through a separate cooler to cool the EGR and remove condensate and other fouling material before being fed at near ambient air temperature to the EGR pump. The condensate and removed fouling material flow back into the hot exhaust gas stream to be exhausted to the ambient atmosphere.